In high voltage systems, it is of great importance that the current through a transmission line can be interrupted in case of a line fault, in order to protect system equipment and system users from damage caused by the fault current. Circuit breakers are therefore provided in order to allow the interruption of a fault current. In direct current (DC) systems, the inductance of a transmission line will only limit the current in the initial transient stage, and the steady state impedance of a transmission line will thus be low. In order to prevent a fault current from growing beyond an acceptable level, a DC circuit breaker is typically connected in series with a large reactor. To maintain system stability and avoid damage to the system, a short breaking time of the DC circuit breaker is desired.
The breaking time of a mechanical DC circuit breaker is largely dependent on the opening time of the mechanical interrupter. Therefore, mechanical interrupters of high opening speed are desired.